OBJECTIVE: To examine whether students use or avoid newly shaded areas created by shade sails installed at schools. DESIGN: Cluster randomised controlled trial with secondary schools as the unit of randomisation. SETTING: 51 secondary schools with limited available shade, in Australia, assessed over two spring and summer terms. PARTICIPANTS: Students outside at lunch times. INTERVENTION: Purpose built shade sails were installed in winter 2005 at full sun study sites to increase available shade for students in the school grounds. MAIN OUTCOME MEASURE: Mean number of students using the primary study sites during weekly observations at lunch time. RESULTS: Over the study period the mean change in students using the primary study site from pre-test to post-test was 2.63 (95% confidence interval 0.87 to 4.39) students in intervention schools and -0.03 (-1.16 to 1.09) students in control schools. The difference in mean change between groups was 2.67 (0.65 to 4.68) students (P=0.011). CONCLUSIONS: Students used rather than avoided newly shaded areas provided by purpose built shade sails at secondary schools in this trial, suggesting a practical means of reducing adolescents' exposure to ultraviolet radiation. TRIAL REGISTRATION: Exempt.